1. Field of the Invention
This invention relates to a device for quantitatively measuring the visual sensation of the sharpness of an image printed on a substrate and more particularly to a method and apparatus for determining an index of the acutance as a quantitative indication of the quality or sharpness of a image for graphic arts films, offpress proofs, press proofs, printing plates, printed press sheets and the like.
2. Description of the Prior Art
In the photographic industry the term "sharpness" has been used to describe the subjective impression produced by the edge of a image in the brain of an observer and the optical density distribution as measured by some objective criterion. However, because sharpness is a psychometric evaluation based on a number of factors an objective measurement of image sharpness has not heretofore been obtainable. The various factors that enter into a determination of sharpness include image register, subject matter, edge enhancement, density range, and resolution.
More recently, the term "acutance" has been used in the photographic industry to distinguish between the subjective impression of sharpness and the objective measurement of the optical density gradient made by a microdensitometric trace across the edge of an image. It has been generally recognized that image sharpness is a subjective concept that can not be objectively measured. Nevertheless, those skilled in the art have felt that a correlation might be formed between the subjective evaluation of image sharpness and acutance measurements made with a microdensitometer.
One known approach is to utilize a series of photographic prints of the same subject but differing degrees of sharpness. The sharpness is manipulated by using different negative materials. Observers then rank the photographic prints according to their sharpness. General agreement is found between the rankings, and the data is manipulated to provide numerical values for the subjectively determined sharpness. However, these efforts overall indicate that there is no satisfactory correlation between image sharpness (acutance) and resolution where resolution is defined as an ovserver's ability to distinguish closely spaced line pairs under adequate magnification. The efforts to find a correlation between the psychometric evaluation of sharpness and an index of acutance calculated from edge density tracings have led to the conclusion that set by the eye for a given viewing condition. However, this approach has indicated that resolving power is not found to furnish consistent information about the perceived sharpness of prints.
Efforts have also been made to examine the effect of sharpness and resolving power on image definition which is regarded in the art as the quality aspect of the photo that is associated with the clarity of detail. Sharpness then by some authorities has been recognized as the impression made on the mind of an observer when examining the boundaries of well resolved elements of detail. In application of these theories, experiments have been conducted utilizing a series of photographic prints on which the graininess and tonal characteristics were held constant by using the same materials in processing for all prints. Sharpness and resolution were varied by changing the distance from the lens to the film. A three-part test subject consisted of a photographic image to be judged for definition, a graduated set of lines for determining resolving power, and a sharp-edge element for measuring acutance. These experiments led to the conclusion that when graininess and tone reproduction are constant and resolving power is adequate to reproduce all the detail that can be observed under the conditions of viewing, acutance correlates well with definition. However, resolution becomes an important component of definition if it drops below the value required to render all the detail that can be observed.
Additional experiments have been attempted in the past to develop means of evaluating and quantifying print quality attributes that ca be distinguished as independent of the subject matter of the image. One approach has been to utilize a star target formed of a circular series of pie-shaped wedges. Star targets are well known for use in measuring the resolution of camera lenses and have proved adequate for measuring the resolution of lithographic printing.
Based on the premise that the observer's impression of sharpness can be predicted from microdensitometric traces across sharp edges in the image, it has been proposed by some authorities that a more complete description of an image could be to determine the ratio of the reflectance changes across various size detail or lines in the original image to their corresponding reflectance change in the lithographic print. With this approach, a scanning microdensitometer is used to determine an edge gradient profile from which a spread function is calculated. Fourier transforms of the spread function are used. The presence of slur, image spread, and doubling had the effect of adding higher harmonics into the sine wave analysis.
The acutance of typed images has also been measured using video camera input. With this approach, acutance is defined as the summation of density differences of pixels in the type in relation to the upper and lower thresholds for density. Edge smoothness is calculated in terms of the standard deviation of pixels from the regression line representing a theoretical sharp edge. Several reproduction systems are compared with respect to their acutance and edge smoothness.
In lithography, various aspects of image structure are related to print quality, where the overall image definition is influenced primarily by resolution and sharpness. Three distinct components of image sharpness have been recognized as including the minor contrast of the printed image, the register accuracy in process color work, and the edge enhancement performed on the image during the color separation process. It is generally recognized that substrates with high brightness and low internal light scatter have greater sharpness. Furthermore, images with higher density range have higher sharpness. Also, it has been found that by increasing ink film thickness, the density range is increased but resolution is lowered.
The register latitude for a given picture varies with the amount of detail and color contrast in the image and with the screen ruling selected for the job. Edge enhancement is a variable and depends on the preferences of the customer, the sharpness of the photographic original and the requirements of the printing system. Optimization conditions are not clear in the case of sharpness. In practice, compromises are necessary between optimum sharpness and the needs for tone and color reproduction.
It is also known to use electronic scanners for making color separations to provide the means for reproducing a considerable range and magnitude of electronic edge enhancement effects through unsharp masking. Three functions are accomplished by unsharp masking. It must correct for the modulation transfer function limitations in originals due to the materials and imaging equipment. It must allow for limitations of the reproduction process. It must also provide a means for editorial change to alter the visual impact of the image. The performance of unsharp masking in electronic scanners is constrained by considerations of resolution, screen, intensity, fringing, and enlargement. Sharpness appearance is influenced by tonal reproduction and frequency response. One approach has been directed to the need for the inclusion of edge effects in the brightness model.
In an effort to measure and evaluate image sharpness, the printing industry has found for some imaging systems that the calculation of acutance from microdensitometric traces has a correlation with perceived sharpness. However, this technique is not available to the printer since it involves specialized equipment. Accordingly, as an alternate means for calculating acutance, the printing industry has developed targets or guides that include line elements which vary in size and frequency. However, the known targets and guides are used solely for the measurement of resolution and not for a quantitative determination of image sharpness.
U.S. Pat. No. 3,393,618 discloses a guide or a control strip for use in the preparation of printing plates in order to determine if the printing operation is running properly. In this context, it is important to determine whether the plate is under developed or over developed, under exposed or over exposed. Then it must be determined once the plate is on the press whether or not the print quality is proper. In an effort to obviate these problems, a control strip in the form of a stencil is placed on a plate by a photographic process, either positive or negative. The control strip is an image bearing item, normally a negative, and is handled by the plate maker so that the image therefrom is located on the plate in a manner designed to measure and inform the plate maker and the pressmen of proper conditions. Preferably, the control strip is located on the edges of the plate so that it can be removed from the resulting print without effecting the print. The control strip has a pattern which is used to determine the condition of the plate and will indicate a gain or loss in image and will indicate any over or under exposure. Reference patterns are located adjacent to each other and the patterns may consist of any geometric design, such as round dots or parallel lines. If the image on the plate is proper then the different reference areas will appear to have equal visual tone. If any over or under exposure or development has taken place in preparing the plate, then the small signal design area of the control strip will change in visual tone proportionately more than the large reference area. This produces a recognizable pattern which informs the plate maker that his plate is improper.
U.S. Pat. No. 3,998,639 discloses a test pattern for determining deviations in the formation of masks used for printing electronic circuit patterns on a photoresist film on a wafer surface. The test pattern includes a grid of lines spaced a preselected distance apart so as to appear as a gray area. The shade of the gray area changes significantly as the width of the lines is altered. To determine if the width of the lines of the test pattern has deviated from a preselected width, a plurality of reference patterns having a preselected deviation from the desired width of line are printed along with the test pattern on the reference plate. If the test pattern has a shade of gray that falls between the two reference patterns, then it is concluded that the test pattern is deemed to be satisfactory because the deviation in the shade of the gray pattern is readily observable.
U.S Pat. No. 4,004,923 discloses a test film target used as a control guide for monitoring developer activity in automatic plate processors to show dot size change in printing and dot size change in bi- and tri- metallic etching. The guide includes a film laid-down on a border or other non-printing area of the negative or positive film and exposed together with the original film to be reproduced on the photosensitized printing plate. Visual observation of the reproduced guide on the plate with the negative eye permits readout of developer solution activity and provide a means for determining when replenisher must be added to the developer solution or a diluent for the developer fluid must be added before any diminution or increase in the strength of the developer has any noticeable effect on the reproduction of the original film. The test film target includes a number of tint areas such as a 1/4 inch circle of 200 line per inch tints. The tints are composed of crossed bands and the width of each band increases in a progressive manner. Since each of the fine tint circles has different band widths, one of the circles will match the background in integrated optical density or tonal effect and therefore, blend together with it. Accordingly, when exposure and development are optimum, the test film will usually blend at a prescribed tint circle. However, when the background does not blend at the prescribed tint circle, an adjustment must be made in the developing process such as adding replenisher to the developing film, if sufficient replenisher is added, the prescribed tint circle will once again blend into the background. With this arrangement, a visual indication of exposure conditions is obtained.
U.S. Pat. No. 4,183,659 is directed to the problem of controlling variations of the thickness of the lines forming the text appearing in a photographically produced brief. The control device in the form of a screen or raster area is printed by photographic means on the photographic print carrier. The screen area may include dots or lines having preselected widths and preselected distance or gaps therebetween. If during the exposure and development of the print carrier, an increase in the thickness of the lines occurs, the spaces or gaps will be filled out and the screen area will turn completely black. On the other hand, in the event a decrease of the thickness of lines occurs, the lines or screen dots in the screen or raster area decrease to an extent that they disappear. Therefore, undesirable broadening or narrowing of the lines in the screen area requires visual detection by the human eye or by means of a densitometer.
U.S. Pat. No. 4,288,157 is a further example of apparatus for controlling the quality of a picture, such as a screened print, processed in a reproduction process. The quality of the screen print is determined by such factors as resolution power, tone displacements, screen dot formation, color layer thickness, and grey balance. In order to evaluate the quality of the screened print based on the above factors, a control device consisting of a number of measurement symbols, such as screen dots, are printed in a preselected array on the picture carrier upon which the printing process is performed. Deformations in the measurement symbols are monitored as the carrier advances through the various stages of the reproduction process. When a deformation occurs in the shape or size of the measurement symbol, the deformation occurs uniformly in all directions during the production process. Therefore, the deformation will become visibly noticeable when a recess or space between the symbols disappears. Upon the disappearance of a recess with a known width, it is concluded that the measurement symbol has reached a certain dimension that has exceeded an acceptable degree of deformation. Then from the value of the known relationship between the original surface of the parts of a measurement symbol of the control device and the size of the recess at such measurement symbol, it is then possible to draw conclusions about the size of the enlargement of the screen dots of the reproduced picture.
U.S. Pat. No. 4,419,426 discloses method and apparatus for visually inspecting the reproduction quality of drawing elements exposed by means of a cathode ray tube on light-sensitive photo material. Visual inspection of print quality to determine whether or not the print is over or under exposed is accomplished by exposing a control field by a cathode ray tube on light-sensitive material. The field includes three different raster points which are combined into a control field. The control field is co-exposed at the beginning and at the end of a text column. If conditions arise during the exposure or development process which result in a change of density and stroke thickness, then an over exposed or under exposed condition can be perceived. If no change in density and stroke thickness occurs, then the control field appears as a neutral gray surface readily identified by the negative eye.
U.S. Pat. No. 4,527,333 discloses a method and apparatus for indicating a quantitative change in dot area of a image in a printing process. The device has particular application in a halftone printing process to quantitatively determine the changes in halftone dot area as an indicator of an increase or decrease in dot area for graphic arts films, prepress proofs, printing plates, printed press sheets, suitable photomechanical processes and the like. The device includes a square matrix array on a substrate. The array comprises a combination of dots and squares where the dots and squares are each equal in size. A dot is positioned in the center of an array with the squares equally spaced around the center dot and the remaining dots positioned at each corner of the square array. The members of the array are spaced a preselected distance apart forming gaps between the adjacent dots and squares. Upon the occurrence of a dot gain, the adjacent sides of the squares touch the dots. As the dot gain increases, the sides of the squares and circumference of the dots merge and overlap so that the gap disappears. With this arrangement, the size of the gap between dots and squares is selected so that the growth of the dots and squares to close the gap will indicate a given dot area gain or dot area loss.
U.S. Pat. No. 4,566,192 is a further example of a device that includes a pattern for determining the dimensions of projected or printed figures. A geometric pattern is used to maintain accurate dimensions during the manufacture of semiconductor wafers.
While it is has been suggested by the prior art devices to evaluate the resolution quality of an image to be reproduced in a printing process, the known devices rely upon visual evaluation of a control device, pattern, measurement element or the like to identify deformations in the reproduced image. The devices depend upon amplitude modulation for an indication of the print quality. This requires that the printed matter be visually inspected to determine whether the quality is acceptable. In the alternative, the reproduced image can be quantitatively analyzed by use of a microdensitometer. However, the use of a microdensitometer requires substantial technical expertise which is generally beyond the capability of the individual who must evaluate the print quality at the production stage.
Therefore, there is need for a device capable of providing a quantitative measurement of the acutance or sharpness of a reproduced image at various stages in the reproduction process. The device must be capable of being used efficiently by the personnel involved at the various stages in the reproduction process.